ctags makes a tags file from the specified C,
Pascal, Fortran, YACC, lex, and Lisp sources. A tags file gives the locations
of specified objects in a group of files. Each line of the tags file contains
the object name, the file in which it is defined, and a search pattern for the
object definition, separated by whitespace.

Using the tags file, a text editor such as
ex(1) or
vi(1) can quickly locate these
object definitions. Indexed objects include subroutines, typedefs, defines,
structs, enums, and unions.

An index of the form expected by vgrind is produced on the standard
output. This listing contains the object name, file name, and page number
(assuming 64 line pages). Since the output will be sorted into
lexicographic order, it may be desired to run the output through
sort(1). Sample use:

ctags produces a list of object names,
the line number and file name on which each is defined, as well as the
text of that line and prints this on the standard output. This is a simple
index which can be printed out as an off-line readable function
index.

Files whose names end in “.c” or “.h” are assumed to
be C source files and are searched for C style routine and macro definitions.
Files whose names end in “.y” are assumed to be
yacc(1) source files. Files whose
names end in “.l” are assumed to be Lisp files if their first
non-blank character is ‘;’,
‘(’, or
‘[’, otherwise, they are treated as lex
files. Other files are first examined to see if they contain any Pascal or
Fortran routine definitions and, if not, are searched for C style definitions.

The tag main is treated specially in C programs. The tag
formed is created by prepending ‘M’ to the name of the file,
with the trailing “.c” and any leading pathname components
removed. This makes use of ctags practical
in directories with more than one program.

Yacc and lex files each have a special tag.
yyparse is the start of the second section of
the yacc file, and yylex is the start of the
second section of the lex file.

The ctags utility is compliant with the
IEEE Std 1003.1-2008
(“POSIX.1”) specification, though its presence is
optional.

The flags [

-BdFuvw

] are
extensions to that specification.

Support for Pascal, YACC, lex, and Lisp source files is an
IEEE Std 1003.1-2008
(“POSIX.1”) extension. The standard notes that
ctags is “not required to
accommodate these languages, although implementors are encouraged to do
so”.

Recognition of functions, subroutines, and procedures for FORTRAN and Pascal is
done in a very simple-minded way. No attempt is made to deal with block
structure; if you have two Pascal procedures in different blocks with the same
name you lose. ctags doesn't understand
about Pascal types.

The method of deciding whether to look for C, Pascal or FORTRAN functions is a
hack.

ctags relies on the input being well formed,
and any syntactical errors will completely confuse it. It also finds some
legal syntax confusing; for example, since it doesn't understand
#ifdef's (incidentally, that's a feature, not a bug),
any code with unbalanced braces inside #ifdef's will
cause it to become somewhat disoriented. In a similar fashion, multiple line
changes within a definition will cause it to enter the last line of the
object, rather than the first, as the searching pattern. The last line of
multiple line typedef's will similarly be noted.